In recent years, as a means for charging electric vehicles (EVs), a non-contact power supplying system of an electromagnetic induction type using coils has been studied. A non-contact power supplying system includes an electricity-feeding side coil (primary coil) to which a power is supplied from an alternating current power source, and an electricity-receiving side coil that is disposed to face the electricity-feeding side coil and is magnetically coupled to the electricity-feeding side coil. In a non-contact power supplying system for electric vehicles, an electricity-feeding side coil is disposed outside the vehicle (floor surface), and an electricity-receiving side coil is disposed inside the vehicle.
As an electricity-feeding side coil and an electricity-receiving side coil, a plane coil that is formed by spirally winding, for example, an enameled wire (a line material configured by covering a conducting body with an insulation film) on one plane is used. A plane coil is manufactured by, for example, fixing an end of the line material to a winding frame, and rotating the winding frame while applying an appropriate tensile force to the line material. When a single-core enameled wire is used for a coil line material, variations of electrical characteristics such as inductance are small, and mass-manufacturing can be achieved in a practical range.
In addition, when a large current of high frequency is required to be supplied to transmit a large power as in the case of a non-contact power supplying system for electric vehicles, a plane coil (hereinafter referred to as “litz wire coil”) formed by winding a litz wire configured by twisting together multiple enameled wires (element wires) is used. One reason for this is that increase in alternating-current resistance due to the skin effect and the proximity effect specific to high frequency can be limited when a litz wire is used.
Conventionally, a litz wire coil has been proposed in which a litz wire is rolled in a tape form in such a manner as to work the wire into a rectangular shape in cross section, and then the wire is spirally wound, thereby increasing the space factor (for example, PTL 1). In the litz wire coil disclosed in PTL 1, the space factor is increased, and therefore the electric resistance is enhanced and the coil size is stabilized, and consequently, variations of inductance can be limited.